Desorption

Introduction

Based on an article by E,. Taglauer in Data Compendium for Plasma-Surface interactions, Nucl. Fusion, Special Issue 1984, IAEA, Vienna (1984)

The surface of the wall material in a fusion device is generally covered with adsorbate layers. These layers can arise from adsorption of the ambient residual gas (O2, H2O, CO, etc.), of the gas used for the plasma (H2, D2, T2) and of impurities segregated at the surface at elevated temperatures (C, S, etc.) or from any other pre-treatment of the material. Adsorbates exist in a variety of states. Physisorption is due to van der Waals forces, with typical binding energies EB ≤ 0.5 eV, occurring (for instance) in multiple adsorption layers. Chemisorption involves binding via exchange or sharing of electrons and has typical binding energies of several electron volts.

The release of adsorbed layers due to an incident flux is often called desorption and sometimes gas sputtering. The physical phenomena involved desorption are in some aspects related to elemental and compound sputtering, but many of the mechanisms and particularly the quantitative data are distinctly different.

Desorption can originate from ion, electron, or photon impact. Ions have the highest yields and the values often much higher than sputtering yields of metals for the same projectiles. Large temperature increases that could cause thermal desorption probably occur only in locally limited areas and hence ion-induced desorption is usually considered in depth.

The role of desorption in impurity release is particularly important in present-day plasma machines but may be less important for fusion reactors. In steady-state operation, segregation of impurities to the hot wall surfaces and subsequent desorption must be considered. Moreover, desorption of plasma particles is important in all stages of the discharge in view of recycling processes.